1. Field of the Invention:
The present invention relates generally to ion mobility spectrometers, and more particularly to ion trap mobility spectrometers and their method of operation for the improved detection of alkaloids, such as narcotics.
2. Prior Art:
Ion mobility spectrometers are used to detect low volatility atmospheric vapor given off, for example, explosives or narcotics. An early ion mobility spectrometer intended for these purposes is shown in U.S. Pat. No. 3,699,333 which issued to Cohen et al in 1972. Improved ion mobility spectrometers are shown in U.S. Pat. No. 5,027,643 and U.S. Pat. No. 5,200,614, which issued to Anthony Jenkins and are assigned to the assignee of the present invention.
The ion mobility spectrometer shown in U.S. Pat. No. 5,200,614, carries a sample vapor into a detector inlet on a carrier gas, such as a stream of air or nitrogen. The carrier gas may be doped with a low concentration vapor (typically a few parts per million) employed as a charge transfer mediator. Sample molecules of interest are fed through an inlet and a diffuser, and into an ionization chamber. A radioactive material, such as nickel.sup.63, or tritium, is disposed in the chamber. Upon passing through the ionization chambers, the ionized sample vapor exits through an open grid into an ion drift region having several field-defining electrodes. A collector electrode or plate is disposed at the end of the drift region of the prior art spectrometer. The grid electrode is normally maintained at the same potential as the walls of the ionization chamber to provide a largely field-free space in which electrons and ion charges build up and interact with the sample molecules under bombardment by the beta-particles from the radioactive walls. Periodically a field is established across the ionization region, for about 0.1-0.2 mS, to sweep the ions into the drift region with the assistance of the switching of the field between electrodes. The ions in the drift region experience a constant electric field, maintained by the annular electrodes. This impels them along the drift region and down toward the collector electrode to be detected and analyzed through their spectra in the prior art spectrometer. After about 0.2 mS the field across the ionization region is again reduced to zero and the ion population is again allowed to build up in the chamber preparatory to the imposition of the next field. The polarity of the fields will be chosen on the basis of whether the detector is operated in a negative or positive ion mode. When detecting explosives, a negative ion mode is usually preferred.
To detect narcotics using ion mobility spectrometers, the sample vapor is typically carried into the detector on a stream of air which may be doped with a low concentration, typically a few parts per million, of nicotinamide vapor as a charge transfer mediator. This dopant compound is well-known in the life sciences as a molecule which exhibits proton affinity, i.e., it acts chemically as a base. Most of the ions produced, by the action of the beta-particles from the radioactive walls on the nitrogen and other gases in the ionization chamber, have a lower proton affinity than nicotinamide so that the positive charge, in the absence of narcotic vapor, is ultimately transferred to the nicotinamide. This has the effect of cleaning up the spectrum obtained from sampling air that is free of narcotic vapors and gives rise to a large single peak in the spectrum which can be used for calibration of the spectrometer.